This invention pertains to pelletized coal and method of injecting same through blast furnace tuyeres. In a conventional blast furnace used for the production of molten iron, the burden charged into the top of the furnace may contain iron oxide, partially reduced iron oxide, coke, limestone and fluxes. Air in the form of a hot blast, which may be enriched with oxygen, in introduced through tuyeres located in the side of the furnace between the smelting zone and hearth section which contains the molten iron.
The coke portion of the burden normally supplies the fuel and an excess of coke provides a porous support for the balance of the burden so that the hot blast entering through the tuyeres can move through the burden and exit through the top of the furnace. The hot blast is essentially heated air and consumes a portion of the coke to provide heat and the combustion products thereof serve to reduce iron oxides and produce molten iron.
The detrimental effect of sulfur on the quality of pig iron produced in the blast furnace is well known. It is also known that the sulfur content of the iron is directly related to the total sulfur contained in the blast furnace burden. Low sulfur is a well known essential requirement for coking coal and this need is a well known contributor to the high cost and short supply of good metallurgical coke. This is true even though the coking process is known to reduce the total amount of sulfur in a coking coal.
It is known to replace a portion of the coke in the burden by introducing other fuels through the blast furnace tuyeres. These other fuels include a wide variety of fluids and solids. In general, fluid fuels such as natural gas, oil, tar, and pitch have been used satisfactorily both individually and in various combinations to reduce the amount of coke in the blast furnace burden. Solid fuels, such as coal, have also been used to reduce the amount of the coke portion of the burden but coal has not had wide acceptance as a replacement for a portion of the coke burden. Reasons for the general unacceptability of blast furnace tuyere coal injection are not completely understood but the following objections to such use of coal have been noted:
(1) Low sulfur is a primary requisite of coking coals for blast furnace use because the total amount of sulfur in the blast furnace burden finds its way into either the slag or molten iron. It is known that the coking of coal removes a portion of the sulfur in the coal so that the metallurgical grade of coke contains less sulfur than the coal.
(2) Known methods for removing sulfur from coal include crushing and separation of coal particles on the basis of size and/or density. It is also known that crushing of some coals results in the production of large amounts of very fine particles which can be detrimental to free flow of fine coal. In addition, the presence of moisture has a variable effect on different coals and moisture may result in the formation of lumps which tend to pack and cause difficulty in the transportation of fine coal. The combination of particle size and moisture content of crushed coal imposes some restriction on the selection of suitable coals for blast furnace injection.
(3) The preferred size of crushed coal for blast furnace injection is about 10 mesh. Large amounts of very fine coal, e.g. 100 mesh, are undesirable because they may form explosive mixtures, are of low density and do not flow well. Coal crushed to sizes coarser than about 10 mesh may be unsuitable for blast furnace injection because combustion is too slow. It is also known that very fine coal is objectionable from the pollution standpoint and fine coals are subject to losses in storage and transit.
(4) The lances used to inject crushed coal through blast furnace tuyeres are exposed to the temperature of the hot blast. These injection lances are known to be subject to plugging which causes erratic blast furnace performance. In addition, rodding to remove plugs from injection lances is dangerous and can add significant labor costs to coal injection.
Examples of prior art patents pertaining to the blast furnace injection of coal include the following:
U.S. Pat. No. 1,349,598 issued Aug. 17, 1920 to L. P. Bosset for "Process of Treating Ores in Blast Furnaces" discloses the injection of fine and coarse carbon (coal) through the tuyeres.
U.S. Pat. No. 3,167,421 issued Jan. 26, 1965 to R. W. Pfeiffer et al for "Powdered Solid Injection Process" is directed to a method for the gaseous injection of solid material such as coal into a reaction vessel such as a blast furnace.
U.S. Pat. No. 3,240,587 issued Mar. 15, 1966 to L. D. Schmidt for "Method of Injecting Particulate Coal into a Blast Furnace" is directed to a method introducing preheated coal into a blast furnace.
None of these prior art patents mention pelletized coal and it has not been previously recognized that the injection of pelletized coal improved the blast furnace operation and the quality of the pig iron.
The principal object of this invention is the injection of pelletized coal so as to improve the operation of a blast furnace.
Another object of this invention is to provide a pelletized coal which is particularly adapted for blowpipe injection through blast furnace tuyeres.
Still another object of this invention is the blast furnace injection of coal pellets containing less sulfur than the coal as mined.
A further object of this invention is to reduce the coke portion of the burden of a blast furnace by providing a portion of the fuel for the furnace by the lance injection of rounded low sulfur pelletized coal through the hot blast tuyeres.
A still further object of this invention is to improve the operation of a blast furnace by replacing a portion of the coke of the burden by lance injecting rounded, low sulfur pelletized coal through the hot blast tuyeres so as to provide a fuel resistant to plugging of injection lances.